Lagrangian particles are used in FDS as water or liquid fuel droplets, flow tracers, and various other objects that are not defined or confined by the numerical mesh. Sometimes the particles have mass, sometimes they do not. Some evaporate, absorb radiation, etc. PART is the namelist group that is used to prescribe parameters associated with Lagrangian particles.

All Lagrangian particles must be explicitly defined via the PART namelist group.

Once a particular type of particle or droplet has been described using a PART line, then the name of that particle or droplet type is invoked elsewhere in the input file via the parameter PART_ID.

There are no reserved PART_ID, all must be defined. For example, an input file may have several PART lines that include the properties of different types of Lagrangian particles:

&PART ID='my tracer', MASSLESS=.TRUE. /

Then these Lagrangian particles can be introduced in the fluid flow from a solid surface via a boundary condition as explained in Section [sec:Injecting-Lagrangian-particles].

WATER=.TRUE. declares that the liquid droplets evaporate into WATER VAPOR, a separate gas phase specie that is automatically added to the calculation by this command. By default, WATER=.FALSE., even though the default properties of droplets are that of water. Setting WATER=.TRUE. instructs FDS to add WATER VAPOR as an explicitly defined specie, and it also invokes appropriate constants related to the absorption of thermal radiation by the water droplets. It also causes the droplets to be colored blue in Smokeview.

For example:

&PART ID='droplets', WATER=.TRUE. /

When a droplet strikes a solid surface, it sticks and is reassigned a new speed and direction. If the surface is horizontal, the direction is randomly chosen. If vertical, the direction is downwards.